1. Field of the Invention
This invention relates to liquid dispensing systems, and more specifically to a catheter-based system for infusing a liquid into the body of a patient, and most specifically to a pain management system which administers a post-operative drug to a wound site of a patient through a catheter that delivers fluid medication uniformly and at a known rate across an infusion section of the catheter.
2. Description of the Related Art
Patient trauma, pain and discomfort resulting from surgery or other procedures is routinely managed through the administration of narcotics or non-narcotic drugs. Narcotics are generally disfavored as a pain management system because they affect the entire physical and mental well-being of the patient rather than only the local physical area of concern. Narcotics also have a variety of undesirable side effects, such as nausea, vomiting, bowel retention, respiratory depression, inhibition of the cognitive process, alteration of appetite, and potentially causing addiction. If used, narcotics can be administered through a variety of known ways, such as intramuscular injection, epidural injection, intravenous injection or orally.
Post-operative pain management is commonly addressed by administering non-narcotic drugs to the patient. Typically, the drug is administered directly into the epidural space of the patient for a period of several days following surgery. However, administering narcotics or non-narcotic drugs into the patient often necessitates monitoring by hospital staff and additional hospital stay due to the side effects of the drugs or because patients cannot be sent home with the required equipment to administer the drugs.
One direct-site drug administration procedure involves using a syringe and needle several times per day to inject the drug at or near the site where the surgeon made the incision through the patient's skin, with several needle pierces made during each dose application. Because many needle pierces are cumulatively made at or near the sensitive incision site, this administration procedure further aggravates patient trauma, pain and discomfort.
Another direct site drug administration procedure involves placing a drug directly into a wound site prior to a surgeon closing the wound. However, this procedure typically lasts only approximately four to six hours and patients often need pain management at a wound site for far in excess of this time period.
A need therefore exists for a pain management system which reduces patient trauma, pain and discomfort resulting from surgery or other procedures. A need also exists for a drug administration system which does not require repeated needle piercings at or near the sensitive incision site. Additionally, there exists a need for a portable drug administration system that a patient can take home to reduce the patient's hospital stay. Finally, there is a need for a dispensing system that dispenses a liquid from a first location to a second location at a predictable and known rate.
In addition to the prior art limitations and needs described above, there are also certain limitations with infusion catheters that are often used in pain management systems. Infusion catheters, which are well known in the art, generally include a flexible hollow tube inserted into some region of the anatomy. The tube typically contains one or more axial lumens within which the fluid may flow. The proximal end of the catheter tube is connected to a fluid source from which fluid is introduced into the catheter tube. The fluid flows within one of the lumens under pressure supplied at the proximal end of the tube. For each lumen, there are commonly provided one or more exit holes along an infusion section near the distal end of the tube, for fluid to exit the tube. Such exit holes may be created by piercing the side wall of the hollow tube.
In certain medical conditions, it is advantageous to deliver fluid medication to a plurality of sites within a wound area. For instance, some wounds which require pain medication may be in communication with many nerve endings, rather than a single nerve trunk. One example of such a wound is a surgical incision. As stated above, it is known to provide a plurality of exit holes through which the fluid medication exits the catheter tube. The exit holes may be provided at various axial and circumferential positions along the catheter tube in order to control the position of the medication delivery sites. An example of a catheter having this configuration is disclosed in U.S. Pat. No. 5,800,407 to Eldor. Also, in some cases it is desirable to deliver such medication under low pressure, so that the fluid is delivered at a relatively low rate. For example, some pain medications must be delivered slowly to avoid toxicity and other side effects. Furthermore, in many cases it is desirable to dispense fluid medication at a substantially uniform rate throughout the infusion section of the catheter, so that the medication is evenly distributed throughout the wound area.
Unfortunately, a limitation of prior art catheters with multiple exit holes, such as the catheter taught by Eldor, is that during low pressure delivery of fluid medication the fluid tends to exit only through the exit hole(s) nearest to the proximal end of the infusion section of the catheter tube. This is because fluids flowing through a tube more readily exit through the exit holes offering the least flow resistance. The longer the flow path followed by the fluid in the lumen, the higher the flow resistance and pressure drop experienced by the fluid. The most proximal holes offer the least flow resistance and pressure drop. Therefore, the fluid tends to exit the catheter tube primarily through these exit holes. As a result, the fluid medication is delivered only to a small region within the wound area. The tendency of the fluid to undesirably flow only through the most proximal exit holes depends upon the hole size, the total number of exit holes, and the flow rate. As the hole size or number of holes increases, the fluid becomes more likely to exit only through the most proximal holes. Conversely, as the flow rate increases, the fluid becomes less likely to do so.
The tendency of the fluid to undesirably exit only through the most proximal holes of the catheter can in some cases be overcome by increasing the flow rate or pressure of the fluid, which causes the fluid to flow through more of the exit holes of the catheter. Indeed, if the flow rate or pressure is sufficiently high, the fluid will flow through all of the exit holes. However, sometimes it is medically desirable to deliver medication at a relatively slow rate, i.e., at a low pressure. Also, even in those cases in which high-pressure fluid delivery is acceptable or desirable, prior art catheters do not provide for uniform fluid delivery along the infusion section of the catheter. Rather, the flow rate through the exit holes nearer to the proximal end of the infusion section tends to be greater than that through the exit holes nearer to the distal end. This is because the fluid passing through the more proximal holes experiences a lower flow resistance and pressure drop. In contrast, the fluid flowing through the more distal holes experiences greater flow resistance and pressure drop, and consequently exits at a lower flow rate. The further distal the hole, the lower the exit flow rate of the fluid. As a result, there is an uneven distribution of medication throughout the wound area.
In another known type of infusion catheter, several lumens are provided within a catheter tube. For each lumen, one exit hole is provided by piercing a hole within the wall of the tube. The exit holes are provided at different axial positions along the infusion section of the catheter tube. In this manner, fluid medication may be delivered to several positions within the wound area. While this configuration offers improved fluid distribution, it has some disadvantages. One disadvantage is that the fluid flow rates through the exit holes are not equal, since the more distal exit holes offer a greater flow resistance for the same reasons discussed above. Another disadvantage is that the number of lumens, and consequently the number of fluid exit holes, is limited by the small diameter of the catheter tube. As a result, fluid may be delivered only to a very limited number of positions within the wound area. Yet another disadvantage is that the proximal ends of the lumens must be attached to a complicated manifold which increases the cost of manufacturing the catheter.
An example of a catheter providing a more uniform dispensation of fluid medication throughout an infusion section of the catheter is illustrated by U.S. Pat. No. 5,425,723 to Wang. Wang discloses an infusion catheter including an outer tube, an inner tube concentrically enclosed within the outer tube, and a central lumen within the inner tube. The inner tube has a smaller diameter than the outer tube, so that an annular passageway is formed therebetween. The outer tube has a plurality of evenly spaced exit holes defining the infusion section of the catheter. In use, fluid flowing within the central lumen passes through strategically positioned side holes within the side walls of the inner tube. In particular, the spacing between adjacent side holes decreases along a length of the inner tube to induce more fluid to pass through the more distal side holes. The fluid then flows longitudinally through the annular passageway before exiting through the exit holes in the outer tube wall. In the annular passageway, the fluid can flow in a distal or proximal direction, depending on the location of the nearest exit hole in the outer tube. This configuration is provided to induce a more uniform exit flow rate of fluid from the catheter.
Unfortunately, the Wang catheter is only effective for relatively high pressure fluid delivery. When used for relatively low pressure fluid delivery, the catheter disclosed by Wang does not provide uniform dispensation of fluid. Instead, the fluid tends to exit through the side holes of the inner and outer tubes that are nearest to the proximal end of the infusion section of the catheter, since these holes offer the least flow resistance. Even for high pressure fluid delivery, there are several limitations of this design. One limitation is that the concentric tube design is relatively complex and difficult to manufacture. Both tubes must be- flexible enough to permit maneuverability through an anatomical system, yet the annular passageway must remain open so that fluid may flow uniformly therein. Another limitation is that the annular passageway may be disturbed if there is a bend in the infusion section of the tube. A bend in the catheter may deform the annular passageway or even cause the inner and outer tubes to come into contact. This can cause an uneven fluid pressure within a longitudinal cross-section of the annular passageway, resulting in non-uniform fluid delivery.
Thus, there exists a need for an improved pain management system, using an improved infusion catheter for delivering fluid medication uniformly along its infusion section. It should come in a relatively simple, easy to manufacture design which is effective for both high flow rate and low flow rate fluid delivery. Furthermore, it is recognized that a particular class of catheters, such as the Wang catheter, may provide uniform fluid delivery only at high fluid pressure or flow rates. However, there is a need for an infusion catheter belonging to this class that has a relatively simple, easy to manufacture design and can maintain uniform fluid delivery while bent or otherwise physically deformed.